In the master branch, this error is swallowed.  I thought it would be better to at least log it.
https://github.com/mjibson/goon/blob/master/goon.go#L275

I think the correct approach is not to return it, since if we return it, the application has to handle it.  Since putMemcache() is only called after a successful datastore Get, I'm not sure why we'd want to throw this back to the application; they got what they wanted.